1. Field of the Invention
This present invention relates to a power conversion circuit for driving fluorescent lamps, such as, for example, cold cathode fluorescent lamps (CCFLs) and more particularly to the drive topology of such circuits.
2. Description of the Related Art
Fluorescent lamps are used in a number of applications where light is required but the power required to generate light is limited. One such application is the backlighting for a notebook computer, or the like. One particular type of fluorescent lamp is a cold cathode fluorescent lamp (CCFL). Such lamps require a high starting voltage (on the order of 700-1,600 volts) for a short period of time to ionize the gas contained within the lamp tubes and fire or ignite the lamp. After the gas in a CCFL is ionized and the lamp is fired, less voltage is needed to keep the lamp on.
CCFL tubes typically contain a gas, such as Argon, Xenon, or the like, along with a small amount of Mercury. After an initial ignition stage and the formation of plasma, current flows through the tube which results in the generation of ultraviolet light. The ultraviolet light in turn strikes a phosphoric material coated in the inner wall of the tube, resulting in visible light.
Power converter circuits such as the LX1580 and LX1581 controllers available from Linfinity Microelectronics Inc. of Garden Grove, Calif. have been used for driving CCFL circuits. Such circuits are designed to drive the CCFL indirectly through a high Q drive circuit (typically on the order of a Q=1 or greater) which preferably includes a resonant circuit and the primary winding of an output transformer. (The Q, or quality, of a circuit is the inductive or capacitive reactance of the circuit at resonance divided by the resistance.) The secondary of the transformer is coupled to the lamp and includes feedback for sensing current flowing through the lamp. Typically, the feedback is provided through a secondary transformer to provide the sensed current to the controller. The resonant circuit is either in a half bridge topology or a full bridge topology using power MOSFETs (metal oxide semiconductor field effect transistors) to provide the DC to AC conversion. Since the drive circuit including the transistors is a high Q resonant circuit, the DC voltage is converted into substantially a sine wave by the resonant circuit.
To control the current through the lamp and to provide dimming capability in the LX1581, the controller circuit typically drives the MOSFETS at a high frequency based upon the resonant frequency of the resonant circuit. Maximum power is provided by switching the MOSFETS at the resonant frequency of the circuit, and the output current can be controlled by changing the frequency of the output drive signals either towards the resonant frequency or away from the resonant frequency.
However, although such circuits represent a dramatic improvement over earlier prior art CCFL circuits, they have certain disadvantages. To attain a high Q circuit, additional capacitors and inductors typically need to be added to the circuit. As a result, this increases the number of external components. Increasing the number of external components increases the total cost and also increases the surface area of a printed circuit board using the product. In portable applications, such as notebook computers and personal digital assistants (PDAs), size may be critical and therefore it is desired to reduce the number of components.